This invention relates generally to diathermy equipment and more particularly to an arrangement which permits two diathermy applicator heads to be used in close proximity without adversely effecting one another.
U.S. Pat. Nos. 3,800,802 and 4,210,152 to Berry disclose diathermy machines which are used to apply RF energy to the human body for therapeutic purposes. As described in these patents, a diathermy treatment head which applies RF energy to the patient produces both electromagnetic and electrostatic energy. Only the electromagnetic energy is useful in therapy. The electrostatic field simply heats the surface area of the skin without deep penetration and is therefore undesirable. Electrostatic shields of the type shown in the additional Berry patent Nos. 4,068,292 and 4,281,362 are normally used to attenuate the electrostatic field.
In order to properly and efficiently utilize the electromagnetic field that is generated by the applicator head, the circuitry should be tuned and the power applied to the patient should be closely controlled at the desired level. As the patient moves or the body temperature or circulation changes, the load can vary rather widely. To compensate for such expected changes in the load, automatic tuning devices are desirable because they continuously maintain the head tuned to a resonant condition to assure maximum power transfer from the applicator head to the patient load.
Although continuously tuned applicator heads have performed in a satisfactory manner for the most part, problems have been encountered when two applicator heads are used in close proximity to one another. This most often occurs when two different areas of the body are to be treated simultaneously by two different applicator head which may or may not be part of the same diathermy machine. The operation of two treatment heads in close proximity results in the two heads "cross talking" or "fighting" with each other. This type of interference is most prevalent when the two heads apply different amounts of power at different pulse rates, although it is also a factor when the heads operate at the same power level and rate. Physically, what happens is that the magnetic field generated by each applicator head interferes with the other head and particularly its power sense and tuning circuits. The interference causes the tuning systems to attempt to retune the applicator heads and continuously "hunt" for a resonant condition. Due to the differences in the phase and frequency of the excitation current, the tuning systems have a tendency to oscillate which causes inefficiencies and other undesirable operating characteristics of the diathermy equipment. The end result is that the output power from each applicator head is different from what it should be for effective therapy. At the same time, energy is inefficiently transferred from the applicator head to the patient due to the untuned state of both applicator heads and the continuous oscillation of the tuning circuits.